Understanding the structure and properties of geological formations may reduce the cost of drilling wells for oil and gas exploration. Measurements are typically performed in a borehole (i.e., downhole measurements) in order to attain this understanding. For example, the measurements may identify the composition and distribution of material that surrounds the measurement device downhole. The accuracy of the identification may depend on how precisely the tool can be located in the borehole.
In sonic cases, the tool is located in a two-part process. In the first part, an ultrasonic transducer, known to those of ordinary skill in the art as a mud transducer, is used to generate acoustic pulses that traverse separate paths, to determine ultrasonic velocity in the drilling fluid (or “mud”), using the time difference between the pulses upon traversing their respective path lengths.
In a second part, the drilling fluid velocity defined by the mud transducer measurement can be used to determine the tool standoff in the borehole, using the time delay between pulse transmission and reception for a pitch-catch transducer. However, mud transducer measurements are impacted by temperature, density, viscosity, and suspended particulates. These elements lead to systematic measurement errors. Moreover, the measured time difference for the pitch-catch transducer is also limited in its accuracy. Thus, two types of measurement errors manifest themselves during tool location operations.